<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .demo {
            width: 0;
            height: 0;
            border-bottom: 100px solid blue;
            border-left: 50px solid transparent;
            border-right: 50px solid transparent;
        }

        .demo2 {
            position: relative;
            margin-top:20px;
            width: 100px;
            height: 100px;
            background: linear-gradient(45deg, #000 50%, transparent 50%);
        }

        .demo3{
            position: relative;
            margin-top:50px;
            width: 100px;
            height: 100px;
            background-color: red;
            clip-path: polygon(50% 0, 100% 100%, 0 100%);
        }

        .demo4{
            background:
                    linear-gradient(45deg, transparent 50%, #fff 50%),
                    linear-gradient(#FFECE8 25%, #FDCDC5 25%, #FBACA3 50%, #F98981 50%, #F76560 75%, #F53F3F 75%);
        }
    </style>
</head>
<body>
<div class="demo"></div>

<div class = "demo2"></div>
<div class = "demo3"></div>
<div class = "demo4"></div>
</body>
</html>